Fibre optic sensors are becoming a well-established technology for a range of applications, for example geophysical applications. Fibre optic sensors can take a variety of forms, and a commonly adopted form is to arrange a coil of fibre around a mandrel. Point sensors such as geophones or hydrophones can be made in this way, to detect acoustic and seismic data at a point, and large arrays of such point sensors can be multiplexed together using fibre optic connecting cables, to form an all fibre optic system. Passive multiplexing can be achieved entirely optically, and a key advantage is that no electrical connections are required, which has great advantage in ‘wet’ applications such as sea bed monitoring.
In certain applications it is desirable to be able to determine the orientation of a particular point sensor, or group of such sensors packaged together (commonly referred to as a 4C package). WO 03/062750 describes an arrangement whereby an electromechanical orientation sensor is included in such a package, and converts its output into a form which can be detected by one or more of the sensors in that package. The package can be self contained, and only optical inputs and outputs to the package are required.
Distributed acoustic sensing (DAS) offers an alternative form of fibre optic sensing to point sensors, whereby a single length of longitudinal fibre is optically interrogated to provide substantially continuous sensing of acoustic/vibrational activity along its length. The single length of fibre is typically single mode fibre, and is preferably free of any mirrors, reflectors, gratings, or change of optical properties along its length. In order to interpret the received signal, the length of the fibre is divided into a plurality of channels for processing purposes.
In distributed acoustic sensing, Rayleigh backscattering is normally used. Due to random inhomogeneities in standard optic fibres, a small amount of light from a pulse injected into a fibre is reflected back from every location along the length of the fibre, resulting in a continuous return signal in response to a single input pulse. If a disturbance occurs along the fibre it changes the backscattered light at that point. This change can be detected at a receiver and from it the source disturbance signal can be estimated. Low noise levels and high discrimination can be obtained using a coherent optical time domain reflectometer (C-OTDR) approach as described above. An alternative approach to DAS is based on heterodyne interferometry. In this approach light which has passed through a given section of fibre is interfered with light that has not. Any disturbance to this section of fibre causes a phase change between the two portions of light that interfere and this phase change can be measured.
Applicant's currently preferred distributed acoustic sensing arrangement operates with a longitudinal fibre up to 40 km in length, and is able to resolve sensed data into 10 m lengths. Each 10 m length can be interrogated to provide real time data simultaneously along the length of the fibre.
Since the fibre has no discontinuities, the length and arrangement of fibre sections corresponding to each channel is determined by the interrogation of the fibre. These can be selected according to the physical arrangement of the fibre and the well it is monitoring, and also according to the type of monitoring required. In this way, the distance along the fibre, or depth in the case of a substantially vertical well, and the length of each fibre section, or channel resolution, can easily be varied with adjustments to the interrogator changing the input pulse width and input pulse duty cycle, without any changes to the fibre
Distributed sensing is therefore able to provide long range, high resolution, high sensitivity monitoring of linear assets, be they pipelines, perimeters, transportation routes or telecommunication networks. A key advantage of this technique is that use can be made of a an unmodified, substantially continuous length of standard fibre, requiring little or no adaptation or preparation for use. pre-existing optic fibres, already in situ can often be used, and readings can be taken at any and all locations along its length, within the operating boundaries of the system in question.
It is desirable however to be able to remotely monitor additional environmental parameters along a linear asset.
JP09270090 describes the detection of information by a number of physical quantity sensors, and the impressing or vibration of an optic fibre in order to represent this information.